Simple switched-mode power supply with current and voltage limitation

ABSTRACT

This invention relates to a primary-controlled switched-mode power supply of the type of a free-running flyback converter, which comprises a transformer with a primary-side winding, a secondary-side winding and at least one auxiliary winding. The switched-mode power supply comprises a primary-side switch, which is connected to the primary-side winding, in order to interrupt a current flow through the primary-side winding, a freely oscillating circuit for the generation of switching pulses, which drive the primary-side switch, and a circuit for generating an image voltage between the terminals of the auxiliary winding, in order to generate an image voltage, which on the primary side forms a voltage to be regulated on the secondary side. In order to provide a switched-mode power supply of this type, which with reduced complexity enables an improved control characteristic and an increased flexibility with regard to the operating parameters, the switched-mode power supply further comprises a time control unit, which is coupled to the primary-side switch such that the duration of a turn-off period of the primary-side switch can be adjusted within a switching cycle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a switched-mode power supply, in particular aswitched-mode power supply with a primary side and a secondary side,which has a transformer with a primary-side winding, a secondary-sidewinding and at least one auxiliary winding. The primary-side winding andthe auxiliary winding are connected to the primary side and thesecondary-side winding is connected to the secondary side. Theswitched-mode power supply comprises a primary-side switch, which isconnected to the primary-side winding, in order to interrupt a currentflow through the primary-side winding, a freely oscillating circuit forthe generation of switching pulses, which drive the primary-side switch,and a circuit for the generation of an image voltage between theterminals of the auxiliary winding, in order to generate an imagevoltage, which on the primary side forms a voltage to be regulated onthe secondary side.

2. Description of the Related Art

Switched-mode power supplies are used in numerous electronic devices togenerate the low direct voltage required for the supply of theelectronic components from a mains voltage. In this respectswitched-mode power supplies have prevailed over conventional powersupplies with mains transformers in many applications, because above acertain power class they exhibit a better efficiency and in particularrequire less space.

The latter is in particular attributable to the fact that instead of themains voltage a high frequency alternating voltage is transformed,which, instead of the usual mains frequency of 50 Hz or 60 Hz, may forexample be in the range from 20 kHz to 200 kHz. Since the requirednumber of windings on the transformer falls inversely proportionally tothe frequency, the copper losses can in this way be significantlyreduced and the actual transformer becomes substantially smaller.

To further optimise the efficiency, in particular primary switched-modepower supplies are known in which the frequency generated on the primaryside of the high frequency transformer by the switch, for example abipolar transistor, is regulated in dependence of the load applied tothe secondary side of the power supply unit in order to regulate thetransferred power. The feedback required for this type of regulation isfor example realised in that a voltage tapped off an auxiliary windingis used as the controlled variable. An appropriate method of controllingthe output current and/or the output voltage is described in EP 1 146630 A2 and takes into account that the same energy is loaded into thetransformer with each pulse. However, the circuit arrangement shown inthis document has the disadvantage of being of comparatively complicatedconstruction, because a relatively complex integrated circuit is used asthe control circuit.

The most inexpensive way of building a switched-mode power supply withelectrical insulation between the primary and secondary sections is witha free-running flyback converter. This type of power supply however, hasprincipally the disadvantage that with low load the switching frequencyincreases noticeably. Consequently, the power loss with no load and withlow loads is high.

An indirect measurement of the output voltage by measuring the voltageon a primary auxiliary winding or the main primary winding is moredifficult with this type of power supply. Due to the induced voltagefrom the stray inductance, a brief voltage overshoot arises, which witha large pulse width can be filtered out in a simple manner, so that itis possible to determine the secondary voltage relatively accurately.With a low load the pulse width however reduces so far that it is hardlypossible to filter out the voltage induced by the stray inductance. Thismeans that the output voltage on low load can only be determined veryinaccurately. An example of this type of simple discrete circuittechnology can be found in the (unexamined) published British patentapplication GB 02379036. In this circuit the use of an optocoupler issuggested to counter the disadvantages of unsatisfactory controlaccuracy. Such an optocoupler, however, increases in turn the complexityand the r costs of the complete switched-mode power supply.

SUMMARY OF THE INVENTION

Therefore the object underlying the present invention is to provide aswitched-mode power supply of the generic type which with reducedcomplexity facilitates an improved control characteristic and anincreased flexibility with regard to the operating parameters.

The object is solved by a switched-mode power supply with the featuresof claim 1. Advantageous further developments of the switched-mode powersupply according to the invention are the subject matter of variousdependent claims.

The present invention is based on the idea that with the aid of a timecontrol unit, which is coupled to the primary-side switch such that theduration of a switch-off period of the primary-side switch can beadjusted, and in particular extended, within one switching cycle, a lowswitching frequency can be retained for a low load and, consequently, anaccurate voltage control and the setting of various output currentcharacteristics are possible. Furthermore, the switched-mode powersupply according to the present invention is constructed from a fewinexpensive components. The switched-mode power supply according to theinvention therefore offers the advantage of low costs with an exactoutput voltage control, low open-circuit input power and the capabilityof usage in extremely variable applications. Finally, the switched-modepower supply according to the invention also has the advantage ofshort-circuit protection.

According to an advantageous embodiment, the time control unit comprisesa control capacitor for controlling the turn-off time of theprimary-side switch by means of its charge current. In this way,speeding up of the turn-on process as well as speeding up of theturn-off process can be achieved. The turn-off period of theprimary-side switch can be extended via the control capacitor in aparticularly simple manner. In this way, the transferred power is setsuch that an almost load-independent output voltage is produced. Thedetection of the output voltage on the primary side is simplified suchthat the transferred energy is the same with each pulse so that arelatively long time is always provided, during which current flows inthe secondary winding. Brief voltage spikes, which arise due to strayinductance, can with the switched-mode power supply according to theinvention be filtered out by means of RC elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated into and form a part of thespecification for the purpose of explaining the principles of theinvention. The drawings are not to be construed as limiting theinvention to only the illustrated and described examples of how theinvention can be made and used. Further features and advantages willbecome apparent from the following and more particular description ofthe invention is illustrated in the accompanying drawings, wherein:

FIG. 1 shows a block diagram of a primary switched-mode power supplyaccording to this invention;

FIG. 2 shows a circuit diagram of a primary switched-mode power supplyaccording to a first embodiment;

FIG. 3 shows a circuit diagram of a switched-mode power supply accordingto a second embodiment;

FIG. 4 shows a circuit diagram of a switched-mode power supply accordingto a third embodiment;

FIG. 5 shows a circuit diagram of a switched-mode power supply accordingto a fourth embodiment;

FIG. 6 shows a circuit diagram of a switched-mode power supply accordingto a fifth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The illustrated embodiments of the present invention will be describedwith reference to the figure drawings wherein like elements andstructures are indicated by like reference numbers.

Referring now to the drawings and in particular to FIG. 1, a schematicblock diagram of a switched-mode power supply according to thisinvention is shown. The alternating voltage U_(IN), which may forexample be the mains voltage, is applied to the input of theswitched-mode power supply 100. In Europe the mains voltage variesbetween 180 V and 264 V alternating voltage and in America between 90 Vand 130 alternating voltage. The input voltage is rectified andstabilised in block 102. In addition, it is ensured that interferencesignals, which are generated in the switched-mode power supply, do notaccess the alternating voltage network. The primary-side winding 110 ofthe isolating transformer 108 and the primary-side switch 104, which isa transistor here, form a series circuit, which is connected to therectified input voltage. The primary-side switch 104 interrupts thecurrent, which flows through the primary-side winding 110, according tothe control signals from the control circuit 106. The switching pulsessupplied from the control circuit to the control input of theprimary-side switch 104 are controlled by block 116, in which thecontrolled variable is generated with the aid of an auxiliary winding114 of the transformer 108. Here, the two signal paths 120 and 122 referto two significant functions of the block 116: Firstly the signal 120“pumps” the control circuit 106 to maintain the free runningoscillation. Secondly, the signal path 122 controls the control circuit106 such that changes in the switching cycle affect the electricalpower, which is supplied to the transformer 108, in the desired manner.

According to the invention, the control circuit 106 contains a timecontrol unit 107 for this, which ensures that the pause periods (or alsothe turn-off times), in which the primary-side switch 104 is open, arematched in length to the required power. The energy, which is suppliedto the transformer during each turn-on phase of the primary-side switch,always remains the same.

The secondary-side winding 112 of the transformer 108 is, as can be seenfrom FIG. 1, connected to a block 118, which generates thesecondary-side voltage U_(OUT) and optionally stabilises it.

In the following the functional principle of the embodiment, drawnschematically in FIG. 1, of the electrically insulated switched-modepower supply according to the invention is explained in more detail.

The control circuit 106 controls the primary-side switch 104 such thatit is brought alternately into the conducting and non-conducting state.Due to the voltage supplied by the block 102, a current always flowsinto the primary-side winding 110 when the primary-side switch 104 is inthe conducting state. A change in the current stores energy in themagnetic field of the transformer 108. When the primary-side switch 104blocks, the energy stored in the magnetic field is released mainlythrough the secondary-side winding 112 and in the block 118, whichgenerates and stabilises the secondary voltage. A small part of theenergy is released through the auxiliary winding 114 into the block 116.This generates an auxiliary voltage as a controlled variable. The energyis released periodically, but due to rectification and filtering anessentially rectified voltage can be generated as an auxiliary voltage.Since the magnetic coupling between the various windings of thetransformer 108 is constant and does not depend on the value of thecurrent or voltage, the value of the auxiliary voltage is proportionalto the value of the secondary voltage and therefore to the value of theoutput voltage.

By means of the time control unit 107 the turn-off period of theprimary-side switch 104 can be set such that the energy fed into thetransformer depends on the output voltage. Therefore, the transferredpower is set such that an almost load-independent output voltage U_(OUT)is produced. The detection of the output voltage on the primary side issimplified such that the transferred energy is the same with each pulseso that a relatively long time is always provided, during which currentflows in the secondary winding 114.

A circuit diagram of a possible embodiment of the switched-mode powersupply according to the present invention is shown in FIG. 2. The mainfeature of this circuit is that the turn-off period of the primary-sideswitch, here the transistor T12, can be extended by the appropriatecontrol of the transistor T11.

After applying of the input voltage U_(IN) to the terminals K11 and K12,the capacitor C15 is charged via the resistors R11 and R12. Withsufficient voltage, a current flows through the resistor R18, thebase-collector junction of the transistor T11, the resistor R20, thebase-emitter junction of the transistor T12, the resistor R23 and thediode D17. Consequently, the primary-side switch T12 is driven open anda current flows through the primary main winding of the transformer W10(terminal 4/terminal 1). On the auxiliary winding of the transformer(terminal 3/terminal 2) a voltage is induced, which causes directfeedback via the capacitor C15, resistor R23 and capacitor C14, speedingup the turn-on process of the primary-side switch T12.

Now the current increases, which flows through the primary-side mainwinding, the primary-side switch T12, the resistor R23 and the diodeD17. Consequently, the voltage also increases, which is dropped acrossthe resistor R23, and therefore also the base-emitter voltage of thetransistor T13. When the base-emitter voltage of the transistor T13exceeds the threshold voltage, the collector-emitter junction of T13becomes conducting and the transistor T12 is consequently turned off.This interrupts the flow of current in the primary-side winding of thetransformer and the voltages on the transformer windings inverse due toself-inductance. An induced current flows both in the secondary-sidewinding as well as in the auxiliary winding.

The current in the secondary-side winding charges the capacitor C100,generating a voltage, which can be used at the output. The current inthe auxiliary winding charges the capacitor C15 via the diode D15 andthe resistor R13 to a voltage, which corresponds to the voltage on thecapacitor C100, as converted via the winding ratio of the auxiliarywinding to the secondary winding. This means that an image of the outputvoltage across the capacitor C100 is generated at the capacitor C15. Thecurrent in the auxiliary winding also causes via the capacitor C14 anacceleration of the turn-off of the transistor T12.

When the voltage across the capacitor C15 is lower than the sum of thethreshold voltages of the diode D16 and the transistor T10, thetransistor T10 is blocked and the transistor T11 is conducting so thatthe capacitor C14 is quickly charged via the series circuit of theresistor R18, transistor T12 and the resistor R20. In this way, theprimary-side switch T12 is turned on again after a short pause andstarts a new cycle.

If the voltage on C15 exceeds the sum of the threshold voltages of thediode D16 and the transistor T10, the transistor T10 becomes conductingand reduces the base current of the transistor T11 such that it limitsthe charging current of the capacitor C14, therefore extending theturn-off period of the primary-side switch T12.

With the illustrated circuit it is therefore possible in a particularlysimple manner to adapt the transferred power to the output voltageindependently of the connected load by setting the turn-off period. Asalready mentioned, the detection of the output voltage is simplifiedsuch that the transferred energy is the same with each pulse so that arelatively long time is always provided, during which current flows inthe secondary winding. Short voltage spikes, which arise due to strayinductances, can be filtered out with appropriately dimensioned RCelements R13, C13, R14, D14, as illustrated in FIG. 3. Consequently, theimage voltage on the capacitor C15 represents a very accurate replicateof the voltage across the capacitor C100.

A limitation of the output current results from the maximum frequencywhich can be set by means of the resistors R18 and R20. This defines themaximum power point. When the maximum power point is exceeded, theoutput voltage falls and therefore also the voltage across the capacitorC15 decreases. Consequently, the current through the resistors R18 andR20 also falls and as a result, the frequency and the transferred powerare reduced. By changing the ratio of the resistance values R18 to R20,the dependence of the output current on the output voltage can be setsuch that different characteristics are possible.

However, the embodiment shown in FIG. 2 still exhibits a dependence ofthe output current on the input voltage, because the delay times on theprimary-side switch T12 cause a maximum primary current dependent on theinput voltage.

This can be countered in that, as shown in FIG. 3 depicting a secondembodiment of the switched-mode power supply according to the invention,a capacitor C17 is connected to the emitter of the primary-side switch.In this case the capacitor C18 can be replaced by a resistor. As for therest, in FIG. 3 components with the same designations as in FIG. 2 aregiven the same reference symbols.

When, with the primary-side switch T12 turned off, the secondary currenthas decreased to zero, a voltage at the level of the output voltageU_(OUT) added to the forward voltage of the diode D100 is present on thesecondary-side winding. The parasitic capacitances are charged with thisvoltage. With the transformer W10, these capacitances form anoscillating circuit and the oscillation, which is caused by the energystored in the parasitic capacitances, can under some circumstances causethe transistor T12 to turn on again prematurely. This in turn leads to abrief control deviation and therefore to an increased ripple on theoutput voltage U_(OUT). To prevent this, the voltage from the auxiliarywinding is, according to the expanded embodiment shown in FIG. 3, passedto the capacitor C14 via a filter formed from the capacitor C13,resistor R14, diode D14 and resistor R13.

Additionally in FIG. 3, a delay element formed by capacitor C16,resistor R21, resistor R22 and capacitor C18 is provided which delaysthe rise of the base-emitter voltage on transistor T13 due to the riseof voltage across the resistor R23. This delay element is not essentialfor the function of the circuit, but it increases the efficiency,because the turn-off process of the transistor T12 is accelerated due tothe phase shift.

According to a further embodiment, which is shown in the form of acircuit diagram in FIG. 4, a second auxiliary winding can be providedfor power control.

The switched-mode power supply shown in FIG. 4 with galvanic separationbetween the primary and secondary sections also represents afree-running flyback converter. With the additional primary-sideauxiliary winding W10 3-6 a negative voltage is generated via theresistor R124 during the turn-on period of the primary-side switch T110at the anode of the diode D119. (A diode can also be used instead of theresistor R124.) Consequently, on the anode of the diode D119 a currentcan be fed with which the turn-on period of the transistor T111 isextended without the turn-off threshold being affected.

In this way control of the turn-off period of the transistor T110 ispossible. This leads to a low switching frequency on low load and thepower loss on open-circuit and on low load is reduced. The secondaryvoltage can be determined relatively accurately with the aid of theprimary auxiliary windings.

A simple voltage limitation may be achieved by means of the diode D120,resistor R129, capacitor C119 and the diode D121. The RC element R125,C118 here filters out the induced voltage spikes from the strayinductance, improving the control characteristics. The resistor R125provides peak current limitation to protect the diode D121.

The parallel circuit of the RC elements C113, R115 and C114, R116provides low-resistance switching of the transistor T111 with relativelylow holding current. Furthermore, due to the combination of a relativelylarge capacitor C114 and a large resistance value R116, the transistorT110 can be turned on with a delay, because the energy in the capacitorC114 is only reduced slowly. In this way, a continuous adaptation of thepause duration to the load occurs.

An improvement in the control characteristics on very low load can beachieved in the illustrated embodiment with the aid of the diode D114,capacitor C117, diode D115 and resistor R120 or the diode D116. Due tothis circuit, the capacitors C113 and C114 are discharged quicker andcharged more slowly. Consequently, very long pause periods are possible,which are automatically extended with increasing output voltage. Thiscircuit also acts as an overvoltage protection and prevents a dangerousrise in the output voltage U_(OUT) with a simple fault.

With the aid of the RC element R114, C116 the induced voltage spikesfrom the stray inductance can be filtered out, whereby the controlcharacteristics can be further improved.

To reduce the dependence of the output current on the output voltage,the turn-on threshold of the transistor T111 can be matched via theresistor R118.

Furthermore, with the aid of the resistor R123 and the diode D118, theturn-on threshold of the transistor T111 can be matched to reduce thedependence of the output current on the input voltage.

Finally, in the embodiment illustrated in FIG. 4 a temperaturecompensation circuit is provided, comprising the transistor T112,resistor R128 and resistor R127, to reduce the temperature dependence ofthe output current.

A further embodiment of the switched-mode power supply according to theinvention is explained in the following with reference to FIG. 5. Here,the functioning principle of the illustrated circuit is the same as thatof the circuits of FIGS. 2 and 3 with the difference that the circuitaccording to FIG. 5 requires substantially fewer components, because thecontrol of the charging current for the control capacitor C213 isrealised in a more simple manner. The turn-off of the primary-sideswitch T12 occurs via a Zener diode D214, which limits the voltage onthe series circuit of the base-emitter junction of the primary-sideswitch T12 and the resistor R220. On reaching the Zener voltage, theflow of current through the transistor T210 cannot increase further andconsequently the voltage on the transformer falls and the directfeedback causes the primary-side switch T12 to turn off quickly.

With reference to FIG. 6 a further embodiment of the switched-mode powersupply according to the invention is now described, in which anadditional optocoupler is used for the feedback of the output voltage tothe primary side. Various circuits for switched-mode power supplies areknown with low open-circuit input powers, which switch off the primarysection of the power supply via an optocoupler with the undercutting ofa defined output power, thereby facilitating a very low input power. Adisadvantage of this known principle is however that the output voltagecomprises a very large ripple voltage on open circuit.

With a switched-mode power supply as shown in FIG. 6 the voltage controlcan be realised using the optocoupler IC10 and a secondary-side controlcircuit. Here, the optocoupler IC10 is controlled such that it conductswhen the control voltage undercuts its limit. In this way, theswitched-mode power supply operates below the control voltage at maximumfrequency, whereby the frequency is limited by a resistor R415 connectedin series with the optocoupler IC10. On reaching the control voltage,the optocoupler IC10 blocks so far that the switching frequency isreduced to the frequency which is required to maintain the controlvoltage on the output. If the optocoupler IC10 is completely blocked,the switching frequency reverts to the minimum frequency at which onlyvery low power is transferred. In this state the power consumed by thecircuit is very low. In this way, it is possible to keep the voltageripple relatively low despite the very low open-circuit input power.

A current limitation can be realised in this case on the secondary sideusing the same optocoupler IC10.

Alternatively, the current limitation can also be realised on theprimary side. Here, a voltage from the auxiliary winding W10 2-3, whichis proportional to the output voltage, is used for the control of theprimary-side switch T12 via the optocoupler IC10 and the series resistorR415. As a result, the charging current of the capacitor C414 reduceswith falling output voltage and the frequency drops. A lower power istransferred and the output current remains almost constant. Variousoutput characteristics are possible through different dimensioning. Onecommon feature is that the short-circuit current is very low, becausethe optocoupler is blocked in the short circuit.

In contrast to known methods in which optocouplers are employed, herethe minimum frequency and therefore the minimum power are achieved witha blocked optocoupler and the maximum frequency is achieved with aconducting optocoupler. The current control is affected by means ofcontrolling the switching frequency dependent on the output voltagewhich is transferred by an auxiliary winding.

If in the time control unit a diode is provided, which limits thecharging current of the control capacitor during the turn-off time ofthe primary-side switch, the charging of the control capacitor may beprevented and the power control via the turn-off duration can befacilitated in a particularly efficient and simple manner.

A controlled charging current for the control capacitor may be obtainedin a particularly effective manner by a charge-current control circuit,which is arranged between the input terminal of the switched-mode powersupply and the control terminal of the primary-side switch.

An oscillation suppression circuit may be provided according to anadvantageous further development of this invention in order to suppressunwanted oscillations in the control circuit of the primary-side switchand to consequently increase the control accuracy.

A phase-shift circuit may be provided for the phase-shifted turn-off ofthe primary-side switch to accelerate the turn-off process of theprimary-side switch and consequently to increase the efficiency of thewhole switched-mode power supply.

According to a further embodiment, the time control unit is formed suchthat a control signal can be deactivated during a turn-on time of theprimary side switch. In this way, variable pauses and constant pulsescan be obtained with a free-running oscillator in a very efficientmanner.

According to an advantageous embodiment the switched-mode power supplyaccording to the invention comprises two primary-side auxiliarywindings, which may also control the turn-off period of the primary-sideswitch. In this way, low switching frequencies at low load and a reducedpower loss on open circuit can be achieved. The secondary voltage can bedetermined relatively accurately on the primary auxiliary windings.

If one of the auxiliary windings is connected to the primary-side switchvia a diode and a transistor, then a current can be fed to the anode ofthe diode to extend the turn-on period of the transistor withoutaffecting the turn-off threshold. During the turn-on period of theprimary-side switch, a negative voltage is generated on the anode of thediode. Alternatively, the series circuit of two diodes or two resistorsmay also be used. An additional resistor may be provided to limit thepeak current for the diode.

If one of the auxiliary windings is connected via a second diode to acapacitor such that same can be charged to the voltage to be regulatedon the secondary side and that, in dependence of the voltage applied tothe capacitor, a current flows through the diode, a resistor, a thirddiode and the base-emitter junction of the transistor, which delays theturn-on of the primary-side switch due to the turn-on period of thetransistor, a voltage-controlled setting of the turn-off period of theprimary-side switch can be obtained. RC elements, which are connected toa control terminal of the primary-side switch and to the first auxiliarywinding, can facilitate relatively low-resistance switching in thecontrol circuit for a relatively low holding current. Due to thecombination of a relatively large capacitor with a large resistancevalue, the primary-side switch can in addition be turned on delayed,because the energy in the capacitor decays only slowly. This facilitatescontinuous adaptation to the load.

An improvement in the control properties with a very low load ispossible with the aid of an overvoltage protection circuit. Due to thiscircuit, the control capacitors are with increasing output voltagedischarged quicker and charged more slowly.

Consequently, very long pause periods are possible, which areautomatically extended with increasing output voltage. This circuit actsas an overvoltage protection and prevents a dangerous rise in the outputvoltage with a simple fault.

According to an advantageous embodiment, the charge-current controlcircuit comprises a first Zener diode, which is connected via a resistorto the base of a control transistor such that the turn-on period of thecontrol transistor delays the turn-on of the primary-side switch. Inthis way, a functioning principle is obtained, which essentiallycorresponds to that described above, whereby however the control of thecharging current for the control capacitor can be realised in a moresimple manner. A significant advantage is a reduced componentrequirement.

Furthermore, the turning-off of the main switch can be affected by aZener diode, which limits the voltage at the series circuit of thebase-emitter junction of the main switch with a resistor. On reachingthe Zener voltage, the current flow through the primary-side switchcannot rise any further. Consequently, the voltage at the transformer isreduced and the direct feedback causes a quick turn-off.

The temperature dependence of the output current can be reduced in asimple manner by means of a temperature compensation circuit.

According to an advantageous further development of this invention, thevoltage control can be realised using an optocoupler and asecondary-side control circuit. Here, the optocoupler is controlled suchthat it conducts when the control voltage undercuts its limit. In thisway, the switched-mode power supply runs at maximum frequency, wherebythe frequency is limited by a resistor connected in series with theoptocoupler. On reaching the control voltage, the optocoupler blocks sofar that the switching frequency is reduced to the frequency which isrequired to maintain the control voltage on the output. If theoptocoupler is completely blocked, the switching frequency goes back tothe minimum frequency at which only very low power is transferred. Inthis state the power taken up by the circuit is very low and it istherefore possible, despite the very low open-circuit input power, tomaintain the voltage ripple relatively low also on open circuit.

A current limitation may be realised in this case on the secondary side,using the same optocoupler used. Alternatively, the current limit canalso be realised on the primary side. Here, a voltage from an auxiliarywinding, which is proportional to the output voltage, is used for thecontrol of the primary-side switch (via the optocoupler and seriesresistor). As a result, the charging current of the control capacitordecreases with falling output voltage and the frequency drops. A lowerpower is transferred and the output current remains, for example, almostconstant. Various output characteristics are possible through differentdimensioning. One common feature is that the short-circuit current isvery low, because the optocoupler is blocked in the short circuit. Apartfrom low costs and an exact output voltage control, this embodiment alsooffers the advantage of a low open-circuit input power and short-circuitprotection.

While the invention has been described with respect to the physicalembodiments constructed in accordance therewith, it will be apparent tothose skilled in the art that various modifications, variations andimprovements of the present invention may be made in the light of theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

In addition, those areas in which it is believed that those ordinaryskilled in the art are familiar have not been described herein in orderto not unnecessarily obscure the invention described herein.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

1. Switched-mode power supply with a primary side and a secondary sideand with a transformer with a primary-side winding, a secondary-sidewinding and at least one auxiliary winding, wherein the primary-sidewinding and the auxiliary winding are connected to the primary side andthe secondary-side winding is connected to the secondary side, aprimary-side switch, which is connected to the primary-side winding inorder to interrupt a flow of current through the primary-side winding, afreely oscillating control circuit for the generation of switchingpulses for controlling the primary-side switch, a circuit for generatingan image voltage between the terminals of the auxiliary winding, inorder to generate an image voltage, which on the primary side replicatesa voltage to be controlled on the secondary side, wherein theswitched-mode power supply further comprises a time control unit, whichis coupled to the primary-side switch such that the duration of aturn-off period of the primary-side switch can be adjusted within aswitching cycle.
 2. Switched-mode power supply according to claim 1,wherein the time control unit comprises a control capacitor, and thatthe turn-off time of the turn-off time of the primary-side switch can beadjusted by the charging current of said control capacitor. 3.Switched-mode power supply according to claim 1, wherein the timecontrol unit comprises a diode, which is arranged between theprimary-side switch and an input terminal of the switched-mode powersupply such that the charging current of the control capacitor can belimited during the turn-off time of the primary-side switch. 4.Switched-mode power supply according to claim 1, wherein the chargingcurrent of the control capacitor can be controlled by a charge-currentcontrol circuit, which is arranged between the input terminal of theswitched-mode power supply and a control terminal of the primary-sideswitch.
 5. Switched-mode power supply according to claim 4, wherein thecharge-current control circuit comprises two amplifiers, which areconnected in series.
 6. Switched-mode power supply according to claim 1,wherein an oscillation suppression circuit is connected to the auxiliarywinding such that unwanted oscillations in the control circuit of theprimary-side switch are suppressed.
 7. Switched-mode power supplyaccording to claim 1, wherein a phase-shift circuit is provided for thephase-shifted turn-off of the primary-side switch.
 8. Switched-modepower supply according to claim 1, wherein the time control unit isadapted to deactivate a control signal during a turn-on period of theprimary-side switch.
 9. Switched-mode power supply according to claim 1,comprising two primary-side auxiliary windings.
 10. Switched-mode powersupply according to claim 9, wherein one of the auxiliary windings isconnected to the primary-side switch via a resistor, a diode and atransistor.
 11. Switched-mode power supply according to claim 10,wherein one of the auxiliary windings is connected via a second diode toa capacitor such that it can be charged to the voltage to be controlledon the secondary side and that, in dependence of the voltage present atthe capacitor, a current flows through the diode, resistor, a thirddiode and the base-emitter junction of the transistor, which delays theturn-on of the primary-side switch due to the turn-on duration of thetransistor.
 12. Switched-mode power supply according to claim 8, whereinthe control circuit comprises an overvoltage protection circuit. 13.Switched-mode power supply according to claim 4, wherein thecharge-current control circuit further comprises a first Zener diode,which is connected via a resistor to the base of a control transistorsuch that the turn-on duration of the control transistor delays theturn-on of the primary-side switch.
 14. Switched-mode power supplyaccording to claim 13, wherein the charge-current control circuitfurthermore comprises a second Zener diode, which is connected inparallel to the series circuit of the base-emitter junction of theprimary-side switch and a resistor connected to the emitter of theprimary-side switch.
 15. Switched-mode power supply according to claim1, further comprising a temperature compensation circuit forcompensating the temperature of the switching threshold of theprimary-side switch.
 16. Switched-mode power supply according to claim1, further comprising an optocoupler for feeding back a secondary-sidevoltage to the primary circuit.
 17. Switched-mode power supply accordingto claim 16, wherein the optocoupler is connected such that with theoptocoupler in a blocking state a minimum power can be transferred andin the conducting state a maximum power can be transferred.
 18. Methodof controlling of the output voltage of a switched-mode power supply byusing an optocoupler for the feedback of a secondary-side voltage, whichis to be controlled, into the primary circuit, whereby the optocoupleris controlled such that it conducts when a specified limit of thesecondary-side voltage to be controlled is undercut.
 19. Methodaccording to claim 18, wherein an output current is controlled byadjusting a switching frequency dependent on the output voltage, whichis transferred by an auxiliary winding.